“Sadly, two babies who died, both born before 28 weeks, had this uncommon infection, and post-mortems confirmed Serratia marcescens as the cause of death.”

The trust has offered its condolences to the families of the two babes which died.

Consultant microbiologist Jeorge Orendi said that after the outbreak the trust reviewed infection prevention practice immediately with the help of the Health Protection Agency and also carried out a deep clean of the unit.

Serratia marcescens is an organism that occurs naturally in the gut and is often not harmful but can be serious for people who are vulnerable to infection, such as premature babies.

Hugh Pennington, professor of bacteriology at the University of Aberdeen, said the case was unusual because hospitals are generally good at keeping them such bugs under control.